Etching is used to form features during silicon micromaching in micro-electromechanical systems ("MEMS"). The success of etching processes are characterized by the etch rate in dimensions of thickness per unit time. The etch rate has dimensions of thickness per unit time. A high etch rate is generally desired. However, too high an etch rate may render a process difficult to control. Common desired etch rates are in units of hundreds or thousands of angstroms per minute. Selectivity is the ratio of the etch rates of silicon referenced to the etch rate of the mask film being patterned on top of silicon. A particular process may be quoted as having a selectivity of 300 to 1, polysilicon over oxide. This means that polysilicon etches 300 times faster than oxide. Another consideration when characterizing etching performance is producing isotropic etches. Isotropic etches, meaning etches having uniform etch properties in all directions, are desirable.
Silicon micromachining in MEMS often uses wet chemical and plasma etching. Wet chemical etching is a purely chemical process having three steps: movement of the etchant species to the surface of the wafer, chemical reaction with the exposed film that produces soluble byproducts, and movement of the reaction products away from the surface of the wafer. Wet etching often yields high selectivity. However, wet etching can have serious drawbacks such as poor process control and excessive particle contamination. Chemical etchants also can cause surface tension effects during drying. The meniscus force of the liquid etchant can drag on the free standing structure thereby sticking to the structure's surface which can induce direct mechanical damage.
Etching in a plasma environment has several advantages when compared to wet etching. Plasmas are easier to start and stop at precise times defining the beginning and end respectively of the etching process as compared with simple immersion wet etching. Plasma etch processes are much less sensitive to small changes in the temperature of the wafer. Plasma etching involves less contaminant and no damage to fragile structures due to surface tension and stiction forces of wet etchants.
Plasma etching is carried out by introducing a feed gas into the chamber. The feed gas is broken down into chemically reactive species by the plasma. These chemically reactive species diffuse to the surface of the wafer and are adsorbed. The species react with the exposed film. The reaction product is desorbed, diffused away from the wafer, and is transported by gas stream out of the etch chamber.
Plasma etch processes obviate many of the wet chemical etch problems. However, plasma etching has limited selectivity over silicon dioxide and nitride. The plasma environment also produces ions and soft x-ray radiation which can damage or have undesirable charging effects on the electronic devices on the substrate.
In "Plasma-less Dry Etching of Silicon with Fluorine-Containing Compounds", Lbbotson et al, J. Appl. Phys., Vol. 56(10), 1984, p. 2939-2942; it was suggested that some fluorine-containing interhalogens such as xenon difluoride can etch silicon spontaneously in the vapor phase.